1. Field of the Invention
The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus and, more particularly, to a developing device of the type having a developer carrier facing an image carrier for carrying a latent image thereon, a supply member for supplying a developer to the developer carrier, a first conveying member for conveying the developer from a storing section to a developing section where the developer carrier and supply member are located, and a second conveying member for conveying the part of the developer not contributed to development in a direction opposite to the conveying direction of the first conveying member.
2. Discussion of the Background
A horizontal developing device is extensively used in the imaging art and has a developing roller, a toner hopper, and an agitator arranged in the substantially horizontal direction. The toner hopper stores toner while the agitator delivers the toner to the developing roller. However, the problem with the horizontal developing device is that the sectional area of the device is greater than that of the image carrier or photoconductive element. This obstructs the free layout of the developing device.
In light of the above, Japanese Patent Laid-Open Publication No. 5-313488, for example, discloses a developing device having a toner conveying member parallel to a developing roller and a supply member, and a toner storing section positioned at the side of the device. Toner is fed from the storing section to the supply member sideways. This kind of scheme enhances the free layout of the device and miniaturizes the entire device even when three or four developing devices are combined as in a color copier or a color printer.
Japanese Patent Laid-Open Publication No. 3-180876 proposes a developing device having a supply member constantly buried in toner in a predetermined amount over the entire axial dimension thereof, so that a uniform toner layer can be formed on the surface of a developing roller. The developing device includes a casing accommodating the supply member and developing roller. The supply member has a cylindrical contour and supplies the toner to the developing roller while in rotation. The developing roller deposits the toner on the surface of an image carrier. A toner container is fluidly communicated to the casing. Toner replenishing means replenishes toner from the toner container to the casing. A dam extends in parallel to the axis of the supply member and divides the casing into a first section for holding the toner replenished by the replenishing means, and a second section for holding the toner over flowing from the first section over the dam. The supply member is disposed in the first section. Recirculating means is provided for recirculating the toner overflowing from the first section from the second section to the first section.
However, in the conventional developing devices described above, only the toner existing in the operation range of the supply member, which conveys the toner from the toner holding section to the developing device, contributes to development. As a result, it is likely that the toner outside the operation range is not used in the developing step and is wasted as so-called dead toner. Moreover, when the toner is continuously returned from the developing section to the storing section, it is apt to sequentially accumulate in the lower portion of the storing section and obstruct the following toner. Finally, this part of the toner will cohere and will not contribute to development forever.